Small satellites are evolving rapidly to augment the functions of larger satellites as well as individually perform some of the tasks such as stereo-imaging and directional communication which were earlier possible only by their larger counterparts. This evolution is gaining ground as attitude control actuators are being developed for smaller classes of satellites. Actuators based on the principle of momentum exchange like reaction wheels, momentum wheels and control moment gyroscopes (CMGs) have multiple spinning wheels or flywheels. The CMG is one such actuator that enables rapid retargeting and precision pointing which are necessary for applications mentioned above. The CMG has flywheels mounted on gimbals which when actuated produces gyroscopic torque that is the control input for the attitude control system (ACS). A large amount of momentum must be stored in these flywheels to be able to produce torques large enough to achieve rapid retargeting. A single flywheel system consists of a uniform flywheel supported by bearings and spun by a brushless DC motor and housed in a sealed casing. A single flywheel assembly 10 in its most basic form is shown in FIG. 1. This system could be used as a reaction wheel or as the flywheel for a CMG.
One of the obvious problems with a flywheel spinning at high speeds is that of imbalance and its effect on the attitude of the satellite. The imbalance introduces a high frequency jitter (equivalent to the angular speed of the flywheel). This drawback is more pronounced in smaller satellites and the task of attitude control of these satellites becomes more challenging; due to their low inertia the satellites are more sensitive to attitude disturbances [2], be it external (e.g., solar winds) or internal (e.g., due to imbalance in flywheel). The high frequency jitter affects imaging systems [3], pointing antennas, and line of sight type instruments and could also excite some of the flexible structures like solar arrays. Usually the external disturbances are non-periodic, of varying magnitude and very low frequency and attitude changes can be corrected using an attitude control system. The effect of attitude jitter on imaging systems have been previously addressed using compensation techniques/mechanisms on the instrument itself (mounting the instrument on additional gimbals for isolation), software techniques, and digital image processing.
The amplitude of the jitter can be minimized by balancing the flywheel, but over a period of time this may be ineffective as the eccentricity may change due to wear in bearings and thermal or structural distortion of the flywheel or the mount [4]. This motivates a need for an onboard real-time jitter compensation technique which can reduce the amplitude of attitude jitter for the lifetime of the satellite. Attitude control systems are critical to the functionality of the satellite and the loss of attitude control due to flywheel failures can render the satellite useless. Flywheel failures can be attributed to bearing damage and motor failures. Having redundancy in the system, without adding significant mass or increasing the complexity of the system may increase the reliability of the system.
Consider a satellite with a single flywheel spinning at an angular velocity ω about its geometric center. The eccentricity due to the location of the center of mass not coinciding with the geometric center causes imbalance in rotation which imparts a force on the satellite through the mounts causing the satellite to rotate about its center of mass [5]. The direction of the imbalance force varies periodically with a frequency equivalent to the rotational speed ω and the magnitude is proportional to both the magnitude of eccentricity and the rotational velocity. This fluctuating imbalance force causes jitter in the attitude of the satellite. The eccentricity could be due to errors in manufacturing processes like non-homogeneity of the material of the flywheel, machining imperfections, bearing clearances and misalignments, and assembly imperfections [5]. These errors can be minimized by balancing the flywheel assembly before installation on the spacecraft although the errors cannot be eliminated due to instruments and equipment limitations. The eccentricity could also develop over a period of time due to thermal distortion, bearing wear and structural deformation of flywheel, bearings, and structures due to their finite stiffness [4]. In such a case the eccentricity cannot be predicted and requires an onboard correction mechanism to compensate for the imbalance.
Various mechanisms have been used to compensate for attitude jitter. Some of them include signal processing techniques, isolation mechanisms for instruments (such as a double gimbaled mount), and use of dampers to minimize the magnitude of jitter. The first technique is instrument dependant and is based on estimation and filtering that may still include noise and erroneous data. The second technique is expensive and requires additional hardware with complex control. Dampers reduce the magnitude of jitter but still leave some residual jitter corresponding to the minimum energy state of the system. Dampers also require additional space on the spacecraft and stowage of viscous damping liquid.
Therefore, there exists a need for a flywheel assembly for minimizing the amplitude of attitude jitter in satellites. It would be further advantageous to provide such a flywheel assembly that is redundant for increasing reliability.